1. Field of the Invention
The present invention generally relates to a permanent magnet motor comprising a rotor having permanent magnets and a stator provided with a multiphase winding, and more particularly to a motor control device and method for controlling the permanent magnet motor by a pulse width modulation (PWM) technique, and a motor drive system including the motor control device.
2. Description of the Related Art
A position sensorless control system is conventionally known as a technique for obtaining a rotational position signal of a rotor from motor voltage, current or the like by computation without use of Hall ICs etc., thereby controlling a brushless DC motor based on the positional signal. Furthermore, when a brushless DC motor is driven via an inverter circuit by a PWM control, a microcomputer incorporated with a functional part specialized for the sensorless motor control has been commercialized. When the aforesaid microcomputer carries out computation for the position sensorless control by software processing, the PWM control by the position sensorless manner can be realized by use of a single microcomputer. For example, a microcomputer executing the above-described control is disclosed by a document, “Application Note” (published by Infineon Technologies AG, Germany, AP08059 XC886/888 CM/CLM, V 10, May 2007) An about 8-bit microcomputer is generally selected for motor control purposes when an increase in costs should be restrained.
However, the computation for execution of position sensorless drive imposes heavy processing load on microcomputer software and accordingly necessitates longer time. Accordingly, the setting of PWM control period is sometimes limited (see FIG. 11 of the above-referenced document, 3. 2. 1 Node 2 Interrupt Disabled, page 12). For example, when a 16-bit microcomputer is used, processing can be carried out at higher speeds In this case, an increase in costs cannot be avoided although the above-described limitation can be overcome.